The present invention relates to video display devices. More particularly, the invention relates to liquid crystal display devices which are capable of video displays in which each element of the display is connected in series with a non-linear element.
In known two-terminal active liquid crystal display matrices which have column electrodes on one substrate, line electrodes on the other substrate, and a layer of liquid crystal material encapsulated in the space therebetween, non-linear elements are disposed between either the liquid crystal material and the column electrodes or the liquid crystal material and the row electrodes to improve the behavior of the display when it is driven. Active matrices including such non-linear elements are described in the following publications:
1. "Varistor-Controlled Liquid-Crystal Displays", D. E. Castleberry. IEEE. ED-26, 1979, pp. 1123-1128;
2. "A 210.times.228 MATRIX LCD CONTROLLED BY DOUBLE STAGE DIODE RINGS", Togashi et al., Television Association Technical Report, ED 782, IPD 86-3, 1984, Japanese Laid Open Application No. 57273/84;
3. "The Optimization of Metal-Insulator-Metal Non-linear Devices for Use in Multiplexed Liquid Crystal Displays," D. R. Baraff et al., IEEE. ED-28, 1981, from pp. 736-739; and
4. LCTV Addressed by MIM Devices K. Niwa et al., SID 84 DIGEST, 1984, pp. 304-307.
In the foregoing publications, several methods for driving active matrices are suggested. All of the driving methods utilize the switching function of non-linear elements depicted herein in FIG. 2 to control the flow of electric current to the liquid crystal display layer.
In FIG. 3, the common line driving waveform C and the data line driving waveform D of Japanese Laid Open Application No. 57273/84 are shown. These waveforms drive the common line and the data line in the same manner as that in conventional time-sharing driving, which is also known as high duty-cycle driving in liquid crystal display. The cross-hatched portion of waveform C-D shows the voltage which is applied to the liquid crystal layer. FIG. 2 shows how the threshold voltage (V.sub.th) is the turning point in the voltage-current characteristic of the series PI diode at which the current increases sharply. Use of the diode assures that the effective voltage applied to the liquid crystal layer during non-selected periods is extremely low, whereby the ON-signal to OFF-signal ratio of the liquid crystal material is improved to obtain high contrast.
To display gray scales in the known method of driving such matrices by high duty cycle driving, the pulse width of the ON-signal in the selected period is controlled by gray-scale data, that is, the width of the pulse is modulated. An example of a conventional data-line driving circuit is shown in FIG. 4 and a chart showing timing of the voltages in the driving circuit is shown in FIG. 5, where a selected time period T corresponds to pulse widths 301 and 302 of FIG. 3. The clock frequency f and the period T are related by the equation f=16/T. A counter 401 (FIG. 4) counts sixteen clock signals f while outputting binary signals Q.sub.0 to Q.sub.3. A grayscale reference pulse-forming circuit 405 decodes the binary signals Q.sub.0 to Q.sub.3 and, in response thereto, generates gray scale reference pulses P.sub.0 to P.sub.3 (FIG. 5). When a unit width is represented as a cycle of f, gray scale reference pulses P.sub.0, P.sub.1, and P.sub.3, respectively, stand for 1/f, 2/f, 4/f, and 8/f. Memory 402 stores digital data which has been converted from analog gray-scale data. In the known circuit, memory 402 has a capacity of four bits. The signals M.sub.0 -M.sub.3 from memory 402 and P.sub.0 -P.sub.3 from gray-scale reference pulse forming circuit 405 are respectively coupled to four AND gates 403', where they are multiplied. The output of each AND gate 403' is fed to an input of multiple-OR-gate 403, which sums the multiplied signals and as shown in FIG. 5, provides selected signals of sixteen levels of duty cycle in dependence on the data stored in memory 402. A pair of gates 404 are controlled by theoutput of gate 403 in normal or in inverted form for transmission as an ON voltage, V.sub.ON, or an OFF voltage, V.sub.OFF, to a row electrode as a data line driving signal. However, when an active-matrix liquid crystal display having non-linear elements is driven by the known high duty-cycle method described above, problems still remain.
FIG. 6 is a symbolic representation of the structure of a picture element in an active-matrix liquid crystal display which has non-linear elements, depicting a non-linear element 603 and a layer of liquid crystal material 604 as connected in series at the intersection of a row electrode 601 and a column electrode 602. The voltages which appear across non-linear element 603 and liquid crystal layer 604 are hereinafter referred to as V.sub.NL to V.sub.LC, respectively. When a data line driving signal from the driving circuit of FIG. 4 is applied to such an element via row electrode 601 and column electrode 602, the voltage which appears across two-terminal non-linear element 603 and liquid crystal layer 604 is shown in FIG. 7. In this example, the gray scale data signal (M.sub.0, M.sub.1, M.sub.2, M.sub.3) is (0, 1, 0, 1). As a result of the non-linear characteristic (FIG. 2) of the non-linear element, the liquid crystal layer is charged by a large flow of current during the periods t.sub.0 and t.sub.1. Since V.sub.NL is large, V.sub.LC increases rapidly. However, during OFF period t.sub.2, even though V.sub.NL is reduced, the liquid crystal layer is not discharged, since V.sub.NL remains less than V.sub.th. Accordingly, V.sub.LC remains substantially level. In period t.sub.3, V.sub.NL is again increased and V.sub.LC increases, stopping at the level where V.sub.NL is equal to V.sub.th. This driving method, however, does not permit the display of gray scale values using pulse-width modulation in which, for example, t.sub.0 ="0", t.sub.1 "1", t.sub.2 ="0" and t.sub.3 ="1" due to the charge holding action of the non-linear element, because V.sub.LC is not reduced in the period t.sub.2.
It is, therefore, difficult to display gray scale in an active matrix display having non-linear elements which are driven with high duty cycles by the known method described above. There is a need, therefore, for a simple method and a circuit embodying the method for driving an active matrix non-linear element which enables the display of gray scale values.